In recent years, a solar cell (solar cell module) has been widely employed as a device directly converting solar energy into electric energy from the viewpoints of the effective use of natural resources and the prevention of environmental pollution. The development of the solar cell is being carried out in terms of productivity and durability.
As shown in FIG. 1, the solar cell is generally prepared by super-posing a transparent front side (light-receiving side) protection material 11 such as a glass plate, a front side (light-receiving side) sealing film 13A, photovoltaic elements (e.g., photovoltaic elements made of silicon) 14, a backside sealing film 13B and a backside protection material (backside covering member) 12, in this order, and degassing them under reduced pressure, and then heating the front side sealing film 13A and the backside sealing film 13B under application of pressure to crosslink and cure the films, whereby they are bonded and combined. In the conventional solar cell, a plurality of photovoltaic elements 14 are connected with each other in order to generate high electric output. Therefore, in order to ensure insulation properties between the photovoltaic elements 14, the photovoltaic elements 14 are sealed with the front side sealing film 13A and the backside sealing film 13B which have high insulation properties.
As the sealing films 13A, 13B, a film made of ethylene-vinyl acetate copolymer (EVA) is heretofore used because of low cost and high transparency and adhesion (for example, Patent Document 1). In addition, in order to ensure mechanical durability of a solar cell and prevent occurrence of corrosion of conductive wires or electrodes in the solar cell caused by permeation of moisture or water, the materials of the solar cell are bonded and combined so as to have high adhesion and bonding strength by cross-linking the EVA film by using a crosslinker.
However, EVA comprising vinyl acetate as the constituent tends to form acetic acid over time because the EVA is hydrolyzed by moisture at high temperature at which the solar cell is installed. The acetic acid may lead to promote occurrence of corrosion of the conductive wires or electrodes in the solar cell. For this reason, sealing films prepared using a resin other than EVA have been developed. As the sealing films using a resin other than EVA, Patent Document 2 discloses a sealing film prepared using an ethylene-unsaturated carboxylic acid copolymer or ionomer thereof, and Patent Document 3 discloses a sealing film prepared using a mixture of ethylene-methyl acrylate copolymer and EVA.